Lubricants are used in tilting-pad journal bearings to support the weight of the shaft under loaded conditions, to provide suitable stiffness and damping when the bearing is incorporated into a rotor system, and to protect the mechanical components from wear [1-3]. Both the mechanical (e.g., viscosity, density) and thermal (e.g., heat capacity, conductivity) properties of the lubricant are important to ensure proper system function. High viscosity and/or density fluids typically provide more stability and protection for the bearing. A lubricant viscosity that is too high can result in undesirable power losses and unacceptably high pad temperatures [4]. The thermal properties of the lubricant must be selected so that friction in the bearing does not lead to temperature build-up and viscosity changes caused by the degradation of the lubricant. Selecting an application-specific lubricant generally solves this trade-off between bearing stability and bearing efficiency. The result can be a bearing that operates optimally in a narrow window of speeds and loads but is less robust if system conditions change [5].
Volatile oil prices and an increased focus on energy efficiency have driven research in more highly efficient lubricants [6]. Gas-lubricated bearings have been developed recently to accommodate the high shaft speeds needed for clean energy processes [7]. Under high speed and temperature conditions, liquid lubricants can degrade and power losses through the bearing can become significant. Under slight pressure, a gas stream can be delivered to the bearing pad to provide a thin film separating the shaft and the pads. Advances in bearing geometry have helped improve the technical feasibility of such systems. Nevertheless, the lower load capacity and low rotor damping of gas-lubricated bearings when compared to oil-lubricated systems will limit their implementation in the practice [8].
Another way to improve bearing efficiency is to deliver ‘tunable’ lubricants, or those that can have their properties adjusted dynamically in response to changing loading or speed conditions [9]. This can be achieved by delivering binary mixtures of lubricants to the bearing and controlling the ratio of the mixture [10]. For example, one lubricant might have high viscosity and the other low viscosity and intermediate viscosities can be achieved by mixing the two. In such a system and method, the lubricant properties can be selected to produce conditions best suited for the application. Such systems would not be easily reversible since liquid-liquid separations, can be tricky and often consume more energy than could be saved using a tunable fluid. For this reason, mixtures of liquid lubricants have not been widely adopted.